The Calvary Coach bus was carrying 39 people and was heading back to the Philadelphia area when it struck the Western Avenue bridge at about 7:30 p.m., state police said. Some passengers were trapped for more than an hour as rescue crews worked to free them.

"The way the roof had collapsed down, you couldn't get off the bus through the front door, so a lot of panic was set in, a lot of shock," said Boston Fire Department spokesman Steve MacDonald. "On the roof, there was a hatch. Firefighters put ladders to the side of the bus, went up, went down the hatch, and it was a tough scene inside."

MacDonald said many of the victims did not know what happened and were simply in shock. He said firefighters are well-trained to handle this type of incident, even though the situation was one they had not seen in some time.

"The adrenaline kicks in. You know, you train, you train and you train for these situations,” MacDonald said. “And then everyone goes to work."

Authorities said the bus did not belong on the road, where a 10-foot height limit is in place and oversized vehicles are not authorized. State police said the driver, Samuel Jackson, 66, of Philadelphia, "failed to heed signs" warning of the height limit, state police said. Jackson was not injured.

“No citations or charges will be filed until we complete additional investigative steps,” State Police spokesman David Procopio said.

Jackson looked down at his GPS and saw the bridge when he looked up but it was too late to avoid hitting it, Ray Talmedge, owner of the Philadelphia-based Calvary Coach Bus company, told WCAU-TV.

Talmedge said Saturday night in an interview that his driver was being interviewed by police. Talmedge, who said he didn't know anything about the road restrictions, said Jackson also drives a school bus.

The students were part of a Destined for a Dream Foundation group, Talmedge said. Officials with the Bristol, Pa.,-based group, a nonprofit that helps underprivileged youth, refused to comment on the crash when reached by phone.

The group's Facebook page said the trip to Harvard was to "visit the campus, sit with the office of cultural advancement, followed by a tour of the campus ... followed by Harvard Square (shopping, eating, site seeing...etc...) This should be a fun time for all!"

State police said 35 passengers were injured, three of them critically. Procopio said many victims were treated and released overnight.

None of those injured was identified, and state police said they did not know how many of the injured were adults and how many were juveniles.

The bus suffered significant damage in the crash. The front part of the roof was pushed in while the center section bowed downward. Photos posted on the Boston Fire Department's website showed firefighters standing on the top of the bus using boards to extract people. The last victim was freed from the bus around 9 p.m., according to the department.

The Massachusetts Bay Transportation Authority sent buses to pick up other passengers and get them out of the frigid temperatures.

Soldiers Field Road curves along the Charles River and passes by Harvard and Boston University. It is a major roadway to the Massachusetts Turnpike.

The road was reopened Sunday after crews removed the bus and repaired guardrails. There was no structural damage to bridge, only cosmetic issues to the structure and the roadway.

Procopio said authorities are in touch with the National Transportation Safety Board who may assist in the investigation.

The circumstances of the crash recalled a similar accident in Syracuse, N.Y., in 2010. The driver of a double-decker Megabus missed his exit and was using a personal GPS to find the bus station. He passed 13 low-bridge warning signs, some with flashing yellow lights, before hitting an overpass. Four people were killed. The driver was acquitted of homicide charges.

In December, a driver who prosecutors said was nearly asleep at the wheel was acquitted of the most serious charges in a crash that killed 15 gamblers returning to New York City from a Connecticut casino in 2011.

Federal officials stepped up enforcement of bus safety regulations last year, closing more than two dozen operations that mostly ferry passengers in the busy East Coast transportation corridor. It was the largest single federal crackdown on the industry.

Buses carry more than 700 million passengers a year in the United States, according to the American Bus Association.

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